[llvm] [CodeGen] Add preliminary plumbing for `samesign` flag (PR #112354)

[Matt Arsenault via llvm-commits]

================
@@ -837,6 +837,8 @@ void MIPrinter::print(const MachineInstr &MI) {
     OS << "disjoint ";
   if (MI.getFlag(MachineInstr::NoUSWrap))
     OS << "nusw ";
+  if (MI.getFlag(MachineInstr::SameSign))
----------------
arsenm wrote:

This test doesn't cover the parsing, see test/CodeGen/MIR/X86/fastmath.mir as an example 

https://github.com/llvm/llvm-project/pull/112354